This invention relates generally to gas analyzers, and more particularly to automatic calibration circuits utilized in gas analyzers.
Photometric systems have been used with success in measuring concentrations of particular gases, or radiation-absorbing agents, in a gas mixture. For example, such systems have been successfully employed in the measurement of ozone concentrations as shown in U.S. Pat. No. 3,812,330 (assigned to the assignee of the present invention) and the references cited therein, which disclosures are incorporated herein by reference. To determine these gas concentrations, most such devices for determining gas concentrations employ Beer's Law. which states: EQU I=I.sub.o e.sup.-.varies.LC
where:
I is the intensity of emergent radiation;
I.sub.o is the intensity of incident radiation;
.alpha. IS THE ABSORPTIVITY OF THE MATERIAL AT A GIVEN FREQUENCY;
L is the internal cell length; and
C is the concentration of the gas to be measured.
Alternatively, it may be seen that EQU I=I.sub.o when C.dbd.O.
these variables can be related to the currents which result from the use of a photometer employing a light source and photodetector by the inclusion of a reference offset current, commonly referred to as span current or I.sub.s. It can be seen that the variables employed in Beer's Law relate directly to the currents generated in a photometer, or more specifically,
I.sub.o is the photometer current when the concentration of the gas to be detected is zero, and
I is the measured photometer current for any given sample. Since I.sub.s is a reference offset current, it can be seen that EQU I=I.sub.o =-I.sub.s when C.dbd.O, EQU i+i.sub.s =O when C.dbd.O.
it is also known in the art that EQU I.sub.o =SGDA.
where S is the intensity of the radiation source;
D is the detector sensitivity;
A is the electrometer gain; and
G is a factor determined from the proportion of light falling on the detector, transmittivity of the optics, background, and other factors.
Difficulties arise because each of these factors may vary with time or temperature. This variation may be described as zero drift. Detection of zero drift is typically made by switching to a reference state in which the gas in the photometer cell does not contain any of the gas to be measured. Correction for zero drift may then be made by adjusting one of these variables until zero drift has been compensated. One device for achieving this is described in the aforementioned U.S. Pat. No. 3,812,330. While the device disclosed in that patent achieved excellent accuracy, the device is complex and expensive. Thus, there has been a need for a simple, inexpensive device for measuring the concentration of a radiation absorbing gas which provides good repeatability and accuracy of measurement.
It is one object of this invention to provide an improved gas analyzer.
It is another object of this invention to provide an improved automatic calibration circuit for use with a gas analyzer.
It is a further object of this invention to provide an automatic calibration circuit to achieve improved span stabilization.
It is another object of this invention to provide an inexpensive automatic calibration circuit for use with a gas analysis device which results in accurate data while enabling long term unattended use.